In-Situ Diagnostic Methods for SOFC G. Schiller, K.A. Friedrich, M. Lang, P. Metzger, N. Wagner German Aerospace Center (DLR), Institute of Technical Thermodynamics, Pfaffenwaldring 38-48, D-70569 Stuttgart, Germany International Symposium on Diagnostic Tools for Fuel Cell Technologies, Trondheim, Norway, June 23-24, 2009
Outline Introduction Electrochemical Impedance Spectroscopy on Stacks Spatially Resolved Measurements: Current density Voltage Impedance Temperature Gas Composition Optical Spectroscopy X-Ray Tomography Conclusion
Investigation of Degradation and Cell Failures Insufficient understanding of cell degradation and cell failures in SOFC Extensive experimental experience is not generally available which would allow accurate analysis and improvements Long term experiments are demanding and expensive Only few tools and diagnostic methods available for developers due to the restrictions of the elevated temperatures
Conventional Test Stand Diagnostics Conventional test stand diagnostics: provide important and essential information about fuel cell performance and behaviour: U(i) characteristics, OCV… EIS on single cells Current interrupt methods Performance degradation with time U(t); i(t) … Cell voltage distribution U stack = U 1 + U 2 + U 3 …. Pressure loss / Gas tighness test Gas utilization measurement Temperature distribution and control
„Sophisticated“ (non-traditional) in-situ Diagnostics Electrochemical impedance spectroscopy on stacks Spatially resolved measuring techniques for current, voltage, temperature and gas composition Optical imaging Optical spectroscopy Acoustic emission detection X-ray tomography
Challenges for EIS for Stack Investigations Large areas (e.g. 100 cm 2 ) lead to high current and low impedances of about 1 mOhm. Electrochemical processes appear at high frequencies (up to 100 kHz) due to the high reaction rates at high temperatures. Stacks generally contain metallic components leading to high frequency disturbances. Contacting of all cells and sensing in specific cells does not account for the voltage distribution in the stack. The sensor wires are at high temperatures: an optimization of the measurement system is not possible during operation. Strong overlapping of electrode processes; evaluation with equivalent circuits can be inaccurate. For system with current > 40 A no commercial equipment available.
Mitigation of EIS Problems Reduction of the high frequency disturbances by optimization of the wiring of the electrical sensing of the SOFC stack. Variation of the operating conditions (gases, temperature) in order to determine the different impedances of the electrode processes Modeling of the spectra by an equivalent circuit. Development of advanced EIS equipment for high currents / high frequencies in corporation with instrument manufacturer (Zahner Elektrik GmbH).
Experimental Set-up for EIS Measurements of Stacks at DLR
Performance of the 5-Cell Short Stack at 750°C (5 H 2 +5 N 2 +3%H 2 O / 20 air (SLPM), 94 h) 1,2 600 Cell 1-4 : @ 3,5V 1.10 V Pstack = 184 W p Cell 5 : 1,0 FU = 37% 1.05 V power density p [mW/cm²] 0,8 400 cell 5 (top) cell voltage U [V] cell 4 U cell 3 0,6 cell 2 cell 1 (bottom) Cell 5: 404 mW/cm² Cell 4: 476 mW/cm² 0,4 200 Cell 3: 447 mW/cm² Cell 2: 472 mW/cm² Cell 1: 415 mW/cm² 0,2 CSZ05-DGF09-CT, 750°C 5H2+5N2+3%H2O / 20air (SLPM) 94h 0,0 0 0 200 400 600 800 current density i [mA/cm²]
Nyquist Plot of one Cell of a 5-Cell Short Stack at Different Current Densities (750°C, 2.5 H 2 +2.5 N 2 / 20 air (SLPM), 142 h) 1,25 0 mA/cm2 5-Zellen Short Stack [CSZ-05-83-CT], cell 5 60 mA/cm2 T=750°C, Zellfl. 84cm² 1 120 mA/cm2 180 mA/cm2 Gas Concentration 240 mA/cm2 0,75 1 Hz 300 mA/cm2 360 mA/cm2 Im Z [Ohmcm²] 420 mA/cm2 0,5 Cathode Anode 6 Hz 80 Hz 0,25 50mHz 0 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 100 kHz 0.14 - 0.17 cm 2 0.55 - 2.2 cm 2 -0,25 -0,5 Re Z [Ohm*cm²]
Equivalent Circuit for the Fitting of the Impedance Spectra C dl (A) C N (A) C dl (C) R Z L R N (A) R p (C) R P (A) Anode Gas Inductivity Ohmic Cathode Concentration
Voltage Losses at one Cell of a 5-cell Short Stack at Different Current Densities (750°C, 2.5 H 2 +2.5 N 2 / 20 air (SLPM), 142 h) 1,2 CSZ-05-83-CT, 750°C 2,5H2+2,5N2 / 21 Air (SLPM) 100 cell 5, 142 h 1,0 Cell5 @ 700mV : 380 mW/cm 2 80 U cell 0,8 voltage loss [mV] cell voltage U [V] U cell Δ U (Anode) Δ U (Cathode) 60 Δ U (Gas Concentr.) 0,6 Δ U (Ohm) 40 0,4 Δ U 20 Contact resistances 0,2 Polarisation resistances 0,0 0 0 50 100 150 200 250 300 350 400 450 current density i [mA/cm²]
Motivation Strong local variation of gas composition, temperature, current density Distribution of electrical and chemical potential dependent on local concentrations of reactants and products Reduced efficiency Temperature gradients Thermo mechanical stress Degradation of electrodes O 2 O 2 O 2 H 2 H O 2 H 2 H O 2
Measurement Setup for Segmented Cells 16 galvanically isolated segments Local temperature measurements Local and global i-V characteristics Local fuel concentrations Local and global impedance measurements Flexible design: substrate-, anode-, and electrolyte-supported cells Co- and counter-flow
Cell design and Testing Station GC measurement Assembly and contacts From a „simple“ cell design All cell concepts with manually controlled Improved contacting features Reliable assembly Impedance measurement Temperature measurement Flexible housing, impedance spectra with reduced interferences
Schematic Lay-out of the Electrical Circuit of the Segmented Cell Configuration Internal cell resistances: Ri,j, Resistances of the wires contacting the anode: equipotential line equipotential line current busbar current busbar RLA,j Resistances of the wires contacting the cathode: RLK,j Only segments 1, 2, 3, 16 are illustrated
OCV Voltage Measurement for Determination of Humidity 13 14 15 16 • Voltage distribution at standard flow rates: fuel gas 9 10 11 12 air • 48.5% H 2 , 48.5% N 2 + 3% H 2 O, 0.08 SlpM/cm² air 5 6 7 8 1 2 3 4 Nernst equation: p RT 0 H 2 O ln U U rev rev zF p p O 2 H 2 Produced water: S4: 0.61%, S8: 0.72%, S12: 0.78%, S16: 3.30%
Variation of Load - Reformate p(i) 100 mA/cm² p(i) 200 mA/cm² p(i) 400 mA/cm² p(i) 435 mA/cm² fu 100 mA/cm² fu 200 mA/cm² fu 400 mA/cm² fu 435 mA/cm² 300,0 90,0 435 435 400 f u 400 250,0 75,0 power density p [mW/cm²] Power density mW/cm 2 200,0 60,0 fuel utilisation fu [%] Fuel utilisation (%) 200 200 150,0 45,0 100,0 30,0 100 100 50,0 15,0 0,0 0,0 Segment 9 Segment 10 Segment 11 Segment 12 Anode supported cell, LSCF cathode, 73,96 cm², gas concentrations (current density equivalent): 54.9% N 2 , 16.7% H 2 , 16.5% CO, 6,6% CH 4 , 2.2% CO 2 , 3.2% H 2 O (0.552 A/cm²), 0.02 SlpM/cm² air
Reformate: Changes of the Gas Composition at 0 mA/cm² 0,3 H2 CO CH4 CO2 H2O 0,25 H 2 Gaskonzentration / % 0,2 Concentration / % Metallic housing, anode substrate, active area 73.78 cm² KS4X050609-7 in Metallischem Gehäuse; Substrat: Anodensubstrat, Anode: 542 µm NiO/YSZ, Electrolyte: 14 µm YSZ + YDC, aktive Zellfläche:73,78 cm²,A: 542 µm NiO/YSZ, E: 14 µm YSZ + YDC, 0,15 K: 28 µm LSCF, Kontaktierung: 30 µm LSP16+Pt3600, Cathode: 28 µm LSCF Operation conditions: 0.10 A/cm² - Anode Integral, Gasflüsse: 0,552 A/cm² Stromdichteäquivalent (54,9% N 2 , 16,7% H 2 , = 5.52 CO 16,5% CO, 6,6%CH 4 , 2,2%CO 2 , 3,2% H 2 0) // 0,08 SlpM/cm² Luft, (54.9% N 2 , 16.7% H 2 , 16.5% CO, 6.6% CH 4 , 2.2% CO 2 , 3.2% H 2 O 800 °C, 0 mA/cm² 0.08 Nlpm/cm² Air, 800°C) 0,1 H 2 O CH 4 0,05 CO 2 0 Segment 9 Segment 10 Segment 11 Segment 12 9 10 11 12
Alteration of the gas composition at 435 mA/cm² 0,3 H2 CO CH4 CO2 H2O 0,25 H 2 Concentration / % H 2 O Gaskonzentration / % 0,2 0,15 CO 2 CO 0,1 0,05 CH 4 0 Segment 9 Segment 10 Segment 11 Segment 12 10 11 12 9
Combined Experimental and Modeling Approach Objectives of the study: Better understanding of the local variations Identification of critical conditions Optimisation of cell components interconnector H 2 H 2 O H 2 /CO CO 2 CH 4 gas z elyt elde anode electrolyte cathode y y O 2 /N 2 N 2 x x interconnector Experiments on single Electrochemical model of Experiments on single Electrochemical model of segmented SOFC local distributions segmented SOFC local distributions
Potential for Optical Spectroscopies a) In situ microscopy b) In situ Raman laser diagnostics Digital CCD camera Imaging Distance microscope spectrograph (resolution1 µm) Heat & radiation shield Lenses/filter Quarz window Transparent 15 cm flow field SOFC Open tube Pulsed Nd:YAG laser (5 mm) (532 nm, 10 ns) Raman spectroscopy Laser Doppler Anemometry (LDA) Particle Image Velocimetry (PIV) Fast-Fourier Infrared (FTIR) Coherent Anti-Stokes Raman Spectroscopy (CARS) Electronic Speckle Pattern Interferometry (ESPI)
Tomography Diagnosis of PEM Fuel Cells in-situ synchrotron radiography neutron tomography in-situ neutron radiography Investigation of water management under operating conditions
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